American Fisheries Society. 153 
learned that its presence in water pumped from a lake during 
the day time indicates that there is little or no oxygen in that 
water. 
Thus the story of the oxygen is of great importance to the 
life of the lake, because during the months of July, August, Sep- 
tember, and the early part of October, all the lower part of Lake 
Mendota is almost uninhabitable by any animal. There are a 
few creatures that live in the mud; there is found there a species 
of clam, (sphaerium) about as large as a pea when fully grown. 
In spite of the fact that the water above it, and around it, is de- 
void of oxygen, that animal survives apparently in a dormant 
condition. We have pumped up the mud from Lake Mendota 
with these animals in it, and kept them under observation in 
sealed bottles, so that the conditions of hfe, so far as tempera- 
ture and oxygen were concerned, were the same as at the bottom 
of the lake. We found that the animals would continue to live, 
although in a dormant condition; while if taken out and placed 
in water which was aerated, the clam would quickly begin to put 
out its siphons and feed. There are also worms found in the 
mud, but the life at the bottom is very decidedly scanty and poor, 
and one which is not fitted to support any considerable amount 
of fish life at any time of the year. 
This story of the oxygen as shown in Lake Mendota is re- 
peated in principle in all lakes; but the details of the story, and 
the effect upon life are very different in different lakes. AIL of 
the lakes get during the late fall, before they freeze, practically 
as much oxygen as they will hold at the temperatures which they 
have reached ; they come out in the spring with substantially that 
amount of oxygen. The formation of the warm surface layer 
comes at dfferent times in different lakes. In the small lakes it 
comes early in the spring, late in April, or early in May. In 
lakes of 20 or 30 acres the warm layer will be formed at that time 
and the bottom water is then cut off from access to the external 
air. In lakes of a mile or two in length this formation comes late 
in May or early in June, and at that time the lower water will 
be cut off. In the larger lakes from 6 to 8 miles long, like Lake 
Mendota, Lake Geneva or Green Lake, the final formation of 
this layer is still further delayed until the latter part of June or 
